In recent years, a vehicle-mounted image recognition system is widely spread which recognizes surrounding vehicles and running lane through a vehicle-mounted camera to prevent an accident from happening. The recognition target objects are diversified, and besides the vehicle, a pedestrian and a road speed sign are included as the recognition target. As a solution, there is a technique of a stereo camera which measures a distance based on the principle of triangulation using two cameras. In the stereo camera, corresponding points of two cameras in a horizontal direction are obtained, and displacement therebetween is set as a disparity. A distance from the subject vehicle to the corresponding points is calculated based on the disparity and is held as a distance image. Then, since cameras are used in the stereo camera, besides the measurement of the distance by the disparity, for example, the object can be recognized using appearance pattern information itself captured by the camera.
PTL 1 discloses a technique of recognizing a sign using the stereo camera. In the technique of PTL 1, an area of the sign is first estimated in the distance image, an approximate size of the sign is estimated from the distance, and then a template matching is performed according to the size so as to recognize the sign.
PTL 2 discloses a vehicle-mounted image recognition system using a plurality of cameras (not limited to the stereo camera) in which, when the object of the detection target cannot be recognized due to a stain of a lens in one camera, a recognition process is performed by another camera to find out the object of the detection target. In the technique of PTL 2, even when the object cannot be recognized in any one of images, the object can be found in the other image, so that detection missing can be prevented.